Self-piercing rivets are used in many different applications and industries. In particular, self-piercing rivets are used in the automotive industry to join and secure multiple sheets of materials. An ongoing trend in the automotive industry is to reduce the weight of vehicles to improve fuel efficiency and meet Corporate Average Fuel Economy (“CAFE”) standards. As a result, modern vehicles have a variety of materials in addition to steel such as composites and aluminum to reduce weight. Thus, self-piercing rivets are used in vehicle production to join sheets of dissimilar materials.
To join dissimilar materials, self-piercing rivets in the automotive industry have a requisite hardness, which can be between 535 HV and 585 HV. These rivets are typically steel that has been austenized and then quenched in a bath to produce a hardened steel rivet with a martensite crystalline structure. These rivets can then be used to join sheets of material. However, one prevailing issue with hardened rivets is that the increased hardness leads to undesirable brittleness.
In practice, multiple sheets of material are positioned adjacent each other for the riveting process, a punch is on one side of the sheets, and a die is on the other side of the sheets. The self-piercing rivet is secured to the punch, which drives the rivet through the sheets and into the die. The die redirects the tail end of the rivet back onto the sheets to secure the rivet and the sheets together. This redirection deforms the tail end of the rivet in a process called flaring. Under some circumstances, flaring the tail end of the rivet causes cracks in the rivet due to the brittleness of the rivet. Cracks lead to integrity issues for the rivet and possibly failure. Therefore, there is a need for a self-piercing rivet that has the requisite hardness and column strength to join dissimilar sheets of materials, but also has qualities that avoid cracking during flaring and subsequent failure.
Another shortcoming of current rivet systems is the need to have many different rivet geometries and die combinations to accommodate the variety of materials that need to be joined in the automotive industry. Thus, there is also a need for a rivet that can join sheets to reduce the number of rivet geometries and die combinations, which reduces costs.